Growth of algae and pathogens in swimming pool water is almost universally controlled by free chlorine at concentrations of from 1 to 3 milligrams per liter (mg/L) at pH from 7.2 to 7.6; lower chlorine concentrations are marginally effective and higher chlorine concentrations are irritating to humans. Consumption of chlorine results in the formation of chloride ion. Chlorine concentration is maintained by adding elemental chlorine and/or chlorine-releasing chemicals to the water. Examples of the latter are sodium hypochlorite, calcium hypochlorite, and trichloro-s-triaizinetrione.
Algae may be controlled by chlorine alone (at pH 7.2 to 7.6) during periods of cold weather and low solar flux, but other compounds are needed to supplement the action of chlorine during periods of warm weather and high solar flux. Such compounds include n-alkyl dimethyl benzyl ammonium chlorides and/or ions of heavy metals such as copper, silver, zinc, tin, nickel and mercury. Monovalent silver ion would be particularly advantageous because it is effective at low concentrations and is harmless to humans; it is not widely used because it is difficult to dissolve silver ion into swimming pool water without precipitating silver chloride. Upon exposure to light, silver chloride precipitate decomposes to elemental silver, a black precipitate that imparts a haze to the water and discolors pool surfaces. On the other hand, aqueous solutions of silver chloride are light-stable.
The problem of dissolving of silver ion into pool water without precipitating silver chloride is well recognized. U.S. Pat. No. 5,149,354 discloses the reaction of soluble monovalent silver salts with sodium gluconate to form water-soluble complexes with silver, which do not readily react with chloride or sulfate in pool water to form silver precipitates. Complexed silver likewise does not readily inhibit algae and pathogens and supplementation with copper metal and cupric ion is mandated. In some instances the copper compounds are substantially the major ingredient in the formulation. U.S. Pat. Nos. 5,073,382, 5,078,902, 5,089,275, 5,098,582 and 5,223,149 disclose the use of divalent and/or trivalent silver ion. Still others (U.S. Pat. Nos. 4,337,136; 4,525,253; 4,680,114; 4,781,805) effect dissolution of silver by passing a controlled electric current across silver-containing electrodes in a cell through which water is passed. U.S. Pat. No. 4,931,078 discloses a procedure in which silver salts are incorporated into molten glass which, when cooled, releases silver ions into water at a rate controlled by the surface area and chemical composition of the solid glass particles.